Type I interferons (IFN-a/p, hereafter referred to as IFN) are a family of cytokines necessary for the stimulation of effective anti-viral host defense. Both Toll Like Receptor (TLR) dependent and TLR- ndependent mechanisms exist for the production of type I IFNs following viral infection. In an emerging picture of TLR-independent responses to viruses, dsRNA or uncapped ssRNA species produced during the course of virus infection are recognized by the intracellular helicases melanoma differentiation antigen [MDA)-5 and retinoic acid inducible gene (RIG)-I, respectively. This event activates production of IFN via a mitochondrial protein, interferon promoter stimulator (IPS)-1, to activate IFN production. Importantly, our data and those of others show that the death-domain (DD)-containing adaptor proteins FADD (Fas-associated protein with death domain) and RIP1 (receptor interacting protein kinase 1) are also essential for optimal signaling by MDA-5, RIG-I and IPS-1. Although essential roles have been established for RIG-I, MDA-5, IPS-1, FADD and RIP1 in innate immune responses to virus infection, the manner in which these key molecules are activated by virus infection to stimulate IFN gene transcription are poorly defined. Indeed, as our data below indicate, there are almost certainly other cellular molecules/co-factors that connect/complex RIG-I and MDA-5 signaling to IPS-1, FADD and RIP-1 to mediate anti-viral innate immune responses. Accordingly, using a yeast two-hybrid screen for FADD-interacting proteins, we have isolated two novel DexD/H box RNA helicases. The first, referred to as DDX-I, was isolated through a two hybrid screen using IPS-1 as a bait. As second helicase, referred to as Fah-1 (for Fadd-associated helicase), has also been identified, through similar screens using FADD as bait. Importantly, our analysis has confirmed that both helicases are required for cellular defense against virus infection. We therefore hypothesize that these helicases may facilitate FADD-dependent innate immune responses, and propose the following Specific Aims: 1.) Characterization of DDX-I and molecular mechanisms of action: We aim to characterize DDX-I, including elucidating interactions with IPS-1, and evaluate the former molecule's importance in host defense against virus infection. 2.) Characterization of FAH-1 and molecular mechanisms of action: We aim to evaluate the significance of FAH-1 in FADD-mediated innate immune regulation and determine this molecules importance in immune mechanisms of viral control.